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46th Lunar and Planetary Science Conference (2015)
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IMPACT HISTORY ON VESTA: PETROGRAPHIC, COMPOSITIONAL AND FUTURE
CHRONOLOGICAL STUDIES OF MELT CLASTS IN H OWARDITES. J.A. Cartwright1,2, D.W. Mittlefehldt3, K.V. Hodges2, and M. Wadhwa1, 1 Center for Meteorite Studies, 1+2School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, 3NASA/Johnson Space Center, Houston, TX 77058. ([email protected])
Introduction: Howardite meteorites are polymict
breccias composed mainly of eucritic and diogenitic
material [1] that likely originate from the surface of the
Asteroid 4 Vesta [2]. They can be separated into two
subtypes: Regolithic, which represent the lithified remains of the active vestan regolith; Fragmental, which
represent simpler polymict breccias [3]. Amongst the
regolithic features observed in the former (e.g., [4-6]),
melt clasts are particularly striking for their appearance
and compositional variability. They range from glassy
spherules to finely crystalline (i.e., devitrified) clasts,
and clasts containing only relict mineral grains to those
containing only phenocrysts. Glasses can be separated
into compositional sub-types including those with low
FeO/MgO ratios (<5) -low alkali glasses, K-rich (K2O
>0.2 wt.%), Na-rich (Na2O >0.6 wt.%) and CaO-rich,
and those with high FeO/MgO ratios (>10) [7, 8].
There is also a distinction to be made between primary
volcanic melt clasts and those produced by impacts.
While [8] suggested that a lack of chemical homogeneity among their studied melt clasts ruled out a primary
volcanic origin, the low siderophile element contents
observed in such clasts suggest less compositional influence from impactors than commonly assumed [7].
Studying the chronology of the impact melt clasts in
howardites can help us to better determine the timing
of impact events on Vesta and the asteroid belt [9].
In this research, we are launching an investigation
into the petrology, composition (major/trace element
and noble gas) and chronology of melt clasts in howardites. We have selected a set of howardites known to
contain large quantities of melt clasts, and have begun
the petrological and compositional studies of these
materials. Once the melt clasts have been fully classified, we aim to perform chronological studies of individual clasts using both the 40Ar/39Ar and Pb-Pb chronometers, as well as determine the noble gas components present. Of particular note, the study will take
advantage of the laser ablation techniques associated
with the noble gas facilities at ASU, which will allow
high-resolution, in-situ analysis of individual clasts.
The broader aim of this work is to ascertain whether the impact flux in the region of the asteroid belt was
similar to that on the Moon. Our understanding of impact events in the inner Solar System relies heavily on
our analyses of lunar meteorites and returned samples,
and there is currently some debate regarding whether
there was a “Lunar Cataclysm” event around ~3.9 Ga,
or the end of an epoch of “Late Heavy Bombardment”
(LHB) at this time (e.g., [10 and refs. therein]). New
and more comprehensive constraints on howardite melt
clast ages may help determine whether the asteroid belt
experienced such a cataclysm or LHB.
Methods: We have selected 11 Antarctic howardites
(EET 87518, EET 87532, EET 99400, GRO 95535,
GRO 95602, LAP 04838, LEW 87004, MIL 05085,
PCA 02014, QUE 94200, QUE 97001), 1 fall (Kapo-
Fig.1: Photomicrographs (PPL) of Kapoeta showing the range
of melt clasts (MC) present as well as eucritic (Euc) or plagioclase clasts (Plag). A) A variety of MCs including those
with relict grains & skeletal grains. B) Large impact MC with
glassy matrix and relict grains. C) Spherule (Sph) and MC.
46th Lunar and Planetary Science Conference (2015)
Fig.2: QUE 94200 and QUE 97001 A) Pyroxene composition diagram. B) BSE of an MC in QUE 94200 with dendritic olivine micro-phenocrysts.
eta) and 2 Saharan finds (NWA 1929, Dhofar 485) for
this study, as these samples are known to contain high
abundances of melt clast material (e.g., [4, 8, 11]). In
addition, a number of these samples have been previously analysed for either 40Ar/39Ar or Pb-Pb chronology [9, 12], noble gases on bulk aliquots [5-6] and/or
major and trace element compositions [4]. Here we
describe our preliminary petrographic and compositional analyses of Kapoeta, QUE 94200, QUE 97001
and EET 87532. Back scattered electron (BSE) images
and pyroxene compositions have been determined using the electron microprobe at NASA/JSC.
Kapoeta: A range of impact melt clasts, most <2mm
in size, have been identified in Kapoeta (Fig. 1). These
clasts display glassy matrices, embayed clast boundaries, and often contain relict, partially resorbed grains
(e.g., Fig. 1B). These textures were formed by hot
melted material mixing with the grains within the regolith following an impact event. Some of the melt clasts
show dendritic/skeletal crystals of olivine or low-Ca
pyroxene and are free of relict grains (Fig. 1A). If
these are of impact origin, the textures suggest almost
complete re-melting of material. However, these clasts
could also be of volcanic origin. A few spherules were
observed (Fig. 1C), which may be volcanic in origin.
QUE 94200 and QUE 97001: These two howardites
are considered to be paired because of similarities in
petrology and bulk composition [4]. A number of melt
clasts were observed in them, including a large spher-
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Fig.3: EET 87532 A) Pyroxene composition diagram. B)
BSE of an impact MC with relict grains.
ule fragment in QUE 94200 (Fig. 2B) that displays
dendritic olivine micro-phenocrysts. These crystals
have a composition of Fo~55, similar to those of coarser
olivine grains in the matrix (Fo56-60). Some of the melt
clasts contain relict grains and are clearly of impact
origin. However, many of the clasts are free of relict
grains, but do have micro-phenocrysts of olivine or
low-Ca pyroxene, and may be of volcanic origin.
EET 87532: Impact melt clasts are present in abundance within EET 87532. Relict grains within these
clasts have compositions within the range observed for
individual coarse pyroxene grains and matrix grains
(Fig. 3A). This howardite also contains abundant ferroan pyroxene fragments that indicate a source of
mafic rocks more evolved than the most Fe-rich basaltic eucrites.
References: [1] Mittlefehldt, D.W. et al. (1998) Rev.
Min. 36: 4.1-4.195. [2] Drake M.J. (2001) MAPS
36:501- 513. [3] Warren P.H. et al. (2009) GCA
73:5918-5943. [4] Mittlefehldt, D.W. et al. (2013)
MAPS 48:2105-2134. [5] Cartwright, J.A. et al. (2013)
GCA 105:395-421. [6] Cartwright, J.A. et al. (2014)
GCA 140:488-508. [7] Barrat, J.A. et al. (2012) LPSC
XLIII (abs. #1438). [8] Singerling, S.A. et al. (2013)
MAPS 48:715-729. [9] Cohen, B.A. (2013) MAPS
48:771-785. [10] Norman, M.D. & Nemchin, A.A.,
(2014) EPSL 388:387-398. [11] Barrat, J.A. et al.
(2009) MAPS 44:359-374. [12] Bogard, D.D. & Garrison, D.H. (2003) MAPS 38:669-710.